Within the past decade, photonic crystals, also referred to as photonic band gap materials [1], have emerged as a new class of materials and devices providing remarkable capabilities of light control and manipulation. The possibility to modify dynamically the geometrical parameters of the photonic crystal structure makes possible the realization of sensitive optical device and thus the 2D photonic crystal technology extremely attractive for the fabrication of physical sensors [1,2,3,4].
Sensors now play important roles in many applications, including automotive safety, homeland security, environmental monitoring and medical equipment. The interest in micro sensors has dramatically increased during the last decade. Micro-electro-mechanical (MEMS) and micro-opto-electro-mechanical systems (MOEMS) technologies have been considerably developed and the expansion of the application field can be expected to grow both in terms of an academic and commercial interests. Various devices have been reported for improved sensing systems.
There is a need for other properties in sensors which are not yet possible in MEMS devices such as fast response and high sensitivity. Sensors based on optics and integrated optics potentially features configurations without electric wires. The clear advantage is the capability to resist harsh environments, immunity to electromagnetic interference, safety in explosive media, and good sensitivity with high dynamic range [4]. Applying photonic crystals which possess unique dispersion characteristics and photonic bandgap, the sizes of the sensor can be dramatically reduced, approximately over three orders of magnitude less than commercial integrated optical Mach-Zehnder interferometers. The nanoscale dimensions of microscale photonic devices allows the fabrication of low weight, compact, dense, and highly parallel sensors [5].
A step towards the goal of providing a sensitive microscale pressure sensor was provided by Biallo et al. [6], who reported a numerical analysis of waveguide-based photonic crystal pressure sensing device incorporating a microcavity. The principle of operation of the device involved the effect of strain on the refractive index of the structure, which was shown to cause the spectral detuning of the microcavity resonance. All theoretical modelling involved a photonic crystal device suspended in air, neglecting any neighbouring structures that could lead to out-of-plane losses.
While this structure succeeds in theoretically demonstrating sensitivity to pressure, practical limitations likely preclude its application in real-world devices. Firstly, the sensitivity of the device depends on a very high quality factor for the microcavity, which is known to be extremely difficult to achieve in microstructured photonic crystal devices due to limitations in material processing technology. Secondly, the device requires a high spectral resolution for the measurement of pressure, which may limit the range of applications for the device.